FIG. 10 shows a heater 13 used for a conventional general indirectly heated cathode. In FIG. 10, an alumina electrical insulating layer 11 is formed by layering alumina particles on a surface of a metal-wire coil 14 by electrophoresis, spraying, or the like and then sintering it. The metal-wire coil 14 is made of tungsten or rhenium-tungsten alloy and is coiled. A metal cap 17 and a sleeve 10 for holding a cathode 9 are provided outside the heater 13. The heater 13 supplies a sufficient amount of heat to the metal cap 17 and the sleeve 10 so that the cathode 9 emits thermoelectrons. The alumina electrical insulating layer 11 on the surface of the metal-wire coil 14 maintains the electric insulation between the sleeve 10 and the metal-wire coil 14. Further, a dark layer 12 made of a mixture of tungsten-alumina particles and alumina particles is provided on the alumina electrical insulating layer 11, thus increasing the heat transfer efficiency from the heater 13 to the sleeve 10.
However, in an indirectly heated cathode provided with a heater having such an alumina electrical insulating layer, thermal stress is concentrated at uneven parts in the alumina electrical insulating layer during sintering and the practical operation. As a result, cracks 16 and deformation of the heater occur easily, thus causing decrease in volume of heat-transfer to a cathode, increase in heater temperature, bad electrical insulation between the heater and the cathode, heater breakdown, or the like. In addition, the operation temperature of the cathode decreases and therefore electron emission decreases, thus affecting the characteristics of a cathode-ray tube.
In order to solve such problems, various methods have been proposed. For instance, there are methods in which an alumina electrical insulating layer is strengthened by mixing a fibrous or whisker-like high-melting inorganic insulator with an inorganic insulator, thus preventing cracks (Japanese Patent Gazette Tokko Sho 44-1775) and on the contrary, porosity in an alumina electrical insulating layer is increased, thus preventing cracks from progressing (Publication of Unexamined Japanese Patent Application Tokkai Sho 60-221925).
However, in the conventional methods mentioned above, there were problems that materials were expensive and when increasing the porosity it was difficult to obtain a uniform alumina electrical insulating layer, thus significantly affecting the defective percentage in manufacturing a heater or damage on a heater after being incorporated into a cathode. Both above-mentioned methods were effective for a heater operated at relatively low temperature (about 1100.degree. C. or less) but caused a short life of a heater operated at high temperature (at least about 1100.degree. C.), for example, in an impregnated cathode.